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Eariier detailed studies of the aging mechanisms of thickfilm resistors revealed anomalous behavior in which certain resistors appeared to be sensitive to the voltages applied to them, but only when they were plastic-encapsulated and not overglazed. These anomalies have been deliberately provoked in a further special study, reported here, of the influence of encapsulation and the effectiveness of overglazing in safeguarding the long-term stability of thick-film resistors. In this paper ruthenium-based thick-film resistors ranging in value from ohms to megohms were finished in a variety of different ways, such as with and without overglaze, with and without laser trim, and with and without plastic encapsulation, and subjected to damp heat and thermal overstress with and without electrical bias. The plastic encapsulaut employed was a particularly active one selected for its properties of nonneutral pH and high ionic content, which are usually regarded as undesirable for electronic component encapsulation but are useful in this instance to exaggerate the effects to be studied.